(a) Field of the Invention
The present invention relates to a semiconductor device having a low-noise ground line and, more particularly, to a structure for reducing the noise propagating through the ground line in a semiconductor device.
(b) Description of the Related Art
A semiconductor device generally has a pair of source lines including a high voltage source line and a ground line for supplying electric power to the functional elements disposed therein. Referring to FIG. 1 which shows a schematic top plan view of a first conventional semiconductor chip, the semiconductor chip is implemented as a SRAM device and formed on a p-type silicon substrate. The semiconductor chip 10 has a scribe region 12 on the outer periphery of the chip 10, wherein a ground line 13 shown by hatching is disposed. In the internal area of the semiconductor chip 10, another ground line 14 shown also by hatching is disposed for supplying a ground potential to the functional elements in the internal area. Both the ground lines 13 and 14 are connected together through bonding pads 14A and 14B.
A high voltage source line 15 is disposed in the internal area for supplying a source potential to the internal circuit, and connected to an external lead frame (not shown) through bonding pads 15A and 15B disposed at the ends of the source line 15, by using a bonding technique. The ground lines 13 and 14 are also connected to the external lead frame through bonding pads 14A and 14B disposed at the ends of the ground lines 13 and 14, by using a bonding technique.
I/O circuit 16 is disposed as a part of the internal circuit adjacent to the scribe region 12 of the semiconductor chip 10. The I/O circuit 16 includes a plurality of output transistors 19A to 19F, which are connected to a common ground line 17 for the I/O circuit 16, source line 18 and corresponding signal lines not shown in the figure. The signal lines are connected to the external lead frame through bonding pads by a bonding technique for outputting corresponding signals from the output transistors. The ground line 17 and the source line 18 are connected to bonding pads 17A and 18A, respectively, which are connected to external lead frame for receiving the ground potential and the source potential.
With the recent increase in the operational speed of the semiconductor device, it has become difficult to obtain a stable operation of the semiconductor chip of FIG. 1 due to the adverse effect of the noise reflected during signal transmission from the output transistors. For reducing the adverse effect, it may be required that the ground line 17 disposed for the output transistors 19A to 19F have a larger width for suppressing the fluctuation of the ground potential on the ground line 17. However, the demand for reduction of the chip size for the semiconductor device in the recent years prevents a large width for the ground line 17, which leads an unstable operation of the semiconductor device.
Referring to FIG. 2 which shows a top plan view of a corner portion of a typical DRAM formed on an n-type semiconductor substrate as a second conventional semiconductor chip 20, a first stage circuit block 25 as a part of the internal circuit is disposed in vicinity of the outer periphery of the semiconductor chip, wherein ground lines 23 and 24 extend parallel to each other. FIG. 3 shows a circuit configuration for the input stage circuit block 25 shown in FIG. 2, wherein p-channel transistors Q1 and Q2 and an n-channel transistor Q3 are connected in series between a high voltage source line Vcc and the ground line. An address terminal connected to the gates of transistors Q2 and Q3 is connected to the bonding pad 27A through the electrostatic discharge element 26A as shown in FIG. 2. If another address terminal is disposed in the first stage circuit block 25, the another address terminal is similarly connected to the bonding pad 27B through the electrostatic discharge element 26B as shown in FIG. 2.
The bonding pads 27A and 27B are respectively connected to the external lead frame by a bonding technique for connecting the address terminals to the external lead frame. The electrostatic discharge elements 26A and 26B are connected to the ground line 24 for protecting the semiconductor device against destruction of the transistors caused by the electrostatic discharge failure. The ground line 23 disposed on the scribe region 22 is connected to the ground line 24 through a bonding pad 24A, which is connected to the external lead frame by a bonding technique. In FIG. 2, the recent increase in the operational speed of the semiconductor device also causes the ground potential of the ground line 24 to fluctuate due to the noise on the ground line 24.